The lvalues returned by the three argument form of substr() used to be a "fixed length window" on the original string.
In some cases this could cause surprising action at distance or other undefined behaviour.
Now the length of the window adjusts itself to the length of the string assigned to it.

As of perl 5.8.2/5.9.0, tied hashes did not return anything useful in scalar context, for example when used as boolean tests:

if (%tied_hash) { ... }

The old nonsensical behaviour was always to return false, regardless of whether the hash is empty or has elements.

There is now an interface for the implementors of tied hashes to implement the behaviour of a hash in scalar context, via the SCALAR method (see perltie). Without a SCALAR method, perl will try to guess whether the hash is empty, by testing if it's inside an iteration (in this case it can't be empty) or by calling FIRSTKEY.

Formats were improved in several ways. A new field, ^*, can be used for variable-width, one-line-at-a-time text. Null characters are now handled correctly in picture lines. Using @# and ~~ together will now produce a compile-time error, as those format fields are incompatible. perlform has been improved, and miscellaneous bugs fixed.

Using substr() on a UTF-8 string could cause subsequent accesses on that string to return garbage. This was due to incorrect UTF-8 offsets being cached, and is now fixed.

join() could return garbage when the same join() statement was used to process 8 bit data having earlier processed UTF-8 data, due to the flags on that statement's temporary workspace not being reset correctly. This is now fixed.

Using Unicode keys with tied hashes should now work correctly.

chop() and chomp() used to mangle UTF-8 strings. This has been fixed.

sprintf() used to misbehave when the format string was in UTF-8. This is now fixed.

The relative ordering of constants that define the various types of SV have changed; in particular, SVt_PVGV has been moved before SVt_PVLV, SVt_PVAV, SVt_PVHV and SVt_PVCV. This is unlikely to make any difference unless you have code that explicitly makes assumptions about that ordering. (The inheritance hierarchy of B::* objects has been changed to reflect this.)

The C preprocessor symbols PERL_PM_APIVERSION and PERL_XS_APIVERSION, which were supposed to give the version number of the oldest perl binary-compatible (resp. source-compatible) with the present one, were not used, and sometimes had misleading values. They have been removed.

The BASEOP structure now uses less space. The op_seq field has been removed and replaced by two one-bit fields, op_opt and op_static. opt_type is now 9 bits long. (Consequently, the B::OP class doesn't provide an seq method anymore.)

Configure now invokes callbacks regardless of the value of the variable they are called for. Previously callbacks were only invoked in the case $variable $define) branch. This change should only affect platform maintainers writing configuration hints files.

The portability and cleanliness of the Win32 makefiles has been improved.

There are still a couple of problems in the implementation of the lexical $_: it doesn't work inside /(?{...})/ blocks and with regard to the reverse() built-in used without arguments. (See the TODO tests in t/op/mydef.t.)

Reimplement the mechanism of lexical pragmas to be more extensible. Fix current pragmas that don't work well (or at all) with lexical scopes or in run-time eval(STRING) (sort, re, encoding for example). MJD has a preliminary patch that implements this.

Fix (or rewrite) the implementation of the /(?{...})/ closures.

Conversions from byte strings to UTF-8 currently map high bit characters to Unicode without translation (or, depending on how you look at it, by implicitly assuming that the byte strings are in Latin-1). As perl assumes the C locale by default, upgrading a string to UTF-8 may change the meaning of its contents regarding character classes, case mapping, etc. This should probably emit a warning (at least).

Introduce a new special block, UNITCHECK, which is run at the end of a compilation unit (module, file, eval(STRING) block). This will correspond to the Perl 6 CHECK. Perl 5's CHECK cannot be changed or removed because the O.pm/B.pm backend framework depends on it.

Study the possibility of adding a new prototype character, _, meaning "this argument defaults to $_".

Make the peephole optimizer optional.

Allow lexical aliases (maybe via the syntax my \$alias = \$foo.

Fix the bugs revealed by running the test suite with the -t switch (via make test.taintwarn).

If you find what you think is a bug, you might check the articles recently posted to the comp.lang.perl.misc newsgroup and the perl bug database at http://bugs.perl.org/ . There may also be information at http://www.perl.org/ , the Perl Home Page.

If you believe you have an unreported bug, please run the perlbug program included with your release. Be sure to trim your bug down to a tiny but sufficient test case. Your bug report, along with the output of perl -V, will be sent off to perlbug@perl.org to be analysed by the Perl porting team.